Process for fabricating semiconductor memory device with high data retention including silicon nitride etch stop layer formed at high temperature with low hydrogen ion concentration

ABSTRACT

A semiconductor memory device such as a flash Electrically Erasable Programmable Read-Only Memory (Flash EEPROM) includes a floating gate with high data retention. A tungsten damascene local interconnect structure includes a silicon nitride etch stop layer which is formed using Plasma Enhanced Chemical Vapor Deposition (PECVD) at a temperature of at least 480° C. such that the etch stop layer has a very low concentration of hydrogen ions. The minimization of hydrogen ions, which constitute mobile positive charge carriers, in the etch stop layer, minimizes recombination of the hydrogen ions with electrons on the floating gate, and thereby maximizes data retention of the device.

BACKGROUND OF THE INVENTION

1. 1. Field of the Invention

2. The present invention generally relates to the art of microelectronic integrated circuits, and more specifically to a process for fabricating a semiconductor memory device with high data retention including a silicon nitride etch stop layer formed at high temperature with a low hydrogen ion concentration.

3. 2. Description of the Related Art

4. A flash or block erase Electrically Erasable Programmable Read-Only Memory (Flash EEPROM) semiconductor memory includes an array of cells which can be independently programmed and read. The size of each cell and thereby the memory are made small by omitting select transistors which would enable the cells to be erased independently. All of the cells are erased together as a block.

5. A memory of this type includes individual Metal-Oxide-Semiconductor (MOS) memory cells, each of which includes a source, drain, floating gate and control gate to which various voltages are applied to program the cell with a binary 1 or 0, or erase all of the cells as a block.

6. Tungsten damascene is a process for fabricating local interconnects which can be advantageously applied to semiconductor devices including flash EEPROMs. The process includes forming an insulator layer of, for example, tetraethylorthosilicate (TEOS) glass over the memory cells, and using Reactive Ion Etching (RIE) to form vertical interconnect holes through the glass down to interconnect areas (source, drain, etc.) of the cells. The holes are filled with tungsten which ohmically contacts the interconnect areas to form the local interconnects.

7. The TEOS etch is conventionally performed using octafluorobutene (C₄F₈) etchant, which also has a high etch rate for silicon. For this reason, a mechanism must be provided for performing the TEOS etch without allowing the etchant to act on the silicon of the underlying interconnect areas.

8. Such a mechanism includes forming a silicon nitride etch stop layer underneath the TEOS layer, and performing the etch in two stages. The first stage is the octafluorobutene etch through the TEOS layer, which terminates at the etch stop layer since octafluorobutene has a low etch rate for silicon nitride.

9. Then, a second RIE etch is performed using fluoromethane (CH₃F), which forms holes through the portions of the etch stop layer that are exposed through the holes in the TEOS layer, down to the interconnect areas of the devices. This is possible because fluoromethane has a high etch rate for silicon nitride, but a low etch rate for TEOS.

10. The structure can be further facilitated by using a silicide technique to increase the conductivity of the interconnect areas of the cells. Siliciding is a fabrication technique that enables electrical interconnections to be made that have reduced resistance and capacitance.

11. The silicide process comprises forming a layer of a refractory metal silicide material such as tungsten, titanium, tantalum, molybdenum, etc. on a silicon interconnect area (source or drain diffusion region) or on a polysilicon gate to which ohmic contact is to be made, and then reacting the silicide material with the underlaying silicon material to form a silicide surface layer having much lower resistance than heavily doped silicon or polysilicon. A silicide surface layer formed on a polysilicon gate is called “polycide”, whereas a silicide surface layer formed on silicon using a self-aligned process is called “salicide”.

12. A problem which has remained unsolved in the fabrication of flash EEPROM memories and other semiconductor device structures is data retention. A flash EEPROM cell is programmed by creating a negative charge (electrons) on the floating gate. The charge should remain until it is deliberately removed by erasing the cell.

13. However, the charge on a conventional flash EEPROM cell which is fabricated using a silicon nitride etch stop layer that is conventionally formed at a temperature of approximately 350° C. has been found to decrease substantially with time. This problem has remained unsolved in the art.

SUMMARY OF THE INVENTION

14. The present invention overcomes the drawbacks of the prior art by overcoming the problem of unsatisfactory data retention in semiconductor devices such as flash EEPROMs which include silicon nitride etch stop layers.

15. In accordance with the present invention, a semiconductor memory device such as a flash Electrically Erasable Programmable Read-Only Memory (Flash EEPROM) includes a floating gate with high data retention.

16. A tungsten damascene local interconnect structure includes a silicon nitride etch stop layer which is formed using Plasma Enhanced Chemical Vapor Deposition (PECVD) at a temperature of at least 480° C. such that the etch stop layer has a very low concentration of hydrogen ions.

17. The minimization of hydrogen ions, which constitute mobile positive charge carriers, in the etch stop layer, minimizes recombination of the hydrogen ions with electrons on the floating gate, and thereby maximizes data retention of the device.

18. These and other features and advantages of the present invention will be apparent to those skilled in the art from the following detailed description, taken together with the accompanying drawings, in which like reference numerals refer to like parts.

DESCRIPTION OF THE DRAWINGS

19. FIGS. 1 to 10 are simplified sectional views illustrating steps of a process for fabricating a semiconductor device according to the present invention; and

20.FIG. 11 is a simplified diagram illustrating a Plasma Enhanced Chemical Vapor Deposition (PECVD) apparatus for practicing the present invention.

DETAILED DESCRIPTION OF THE INVENTION

21. The present inventors have discovered that a major cause of poor data retention in semiconductor devices such as flash EEPROMs as presented above is a high concentration of hydrogen ions in the silicon nitride etch stop layers of the devices. These hydrogen ions are highly mobile positive charge carriers which migrate to the floating gates of memory cells to recombine with electrons thereon and dissipate the charges on the floating gates.

22. The present invention overcomes these problems, and provides a semiconductor structure including a silicon nitride layer having a low concentration of hydrogen ions. Although the present invention is especially suited to a memory device including a floating gate in which data retention is a problem, the present invention is not so limited, and can be advantageously applied to a large variety of semiconductor devices which may or may not include floating gates or other charge retention elements. For example, the invention may be applied to semiconductor structures which include silicon nitride encapsulation layers.

23. FIGS. 1 to 10 are simplified sectional diagrams illustrating a process for fabricating a portion of a flash EEPROM semiconductor memory device according to the present invention. The detailed configuration of the device is not the particular subject matter of the invention, and only those elements which are necessary for understanding the invention will be described and illustrated.

24. As viewed in FIG. 1, a flash EEPROM memory 10 includes a silicon semiconductor substrate 12. Two erasable memory cells 14 are formed on a surface 12 a of the substrate 12, each including a MOS transistor structure having a source 14 a, drain 14 b, gate oxide layer 14 c, and channel 14 d underlying the gate oxide layer 14 c. The cells 14 are physically and electrically isolated from each other by field oxide regions 16.

25. A polysilicon control gate 14 e is formed over each gate oxide layer 14 c, and a polysilicon floating gate 14 f is formed underneath the control gate 14 e in the gate oxide layer 14 c.

26. Although the gate oxide layers 14 c are shown as being integral, they may comprise two or more sublayers. For example, portions of the gate oxide layers 14 c which underlie the floating gates 14 f may be separate tunnel oxide layers. Further shown in the drawing are electrically insulating gate sidewall spacers 14 g.

27. The construction and operation of the memory 10 are not the particular subject matter of the invention and will not be described in detail. Furthermore, the reference numerals designating the individual elements of the memory cells will be omitted in FIGS. 2 to 10 except as required for understanding the invention to avoid cluttering of the drawings.

28.FIG. 1 illustrates the initial steps of the present process, which consist of providing the substrate 12, and forming semiconductor devices such as the erasable memory cells 14 on the surface 12 a of the substrate 12.

29.FIG. 2 shows how interconnect areas are formed for the elements of the cells using a silicide technique to increase the electrical conductivity. The process comprises forming a layer of a refractory metal silicide material such as tungsten, titanium, tantalum, molybdenum, etc. on the source, 14 a, drain 14 b, and control gate 14 e to which ohmic contact is to be made, and then reacting the silicide material with the underlaying silicon material to form silicide source interconnect areas 18 a, drain interconnect areas 18 b, and control gate interconnect areas 18 c respectively.

30.FIG. 3 illustrates how a silicon nitride (S₃N₄) etch stop layer 20 is formed over the surface 12 a of the substrate 12 and the devices 14 in accordance with the present invention. The etch stop layer 20 is preferably formed using Plasma Enhanced Chemical Vapor Deposition (PECVD) at a temperature of at least approximately 480° C. to a thickness of approximately 800± 50 Å.

31. Basic Chemical Vapor Deposition (CVD) is a technique which normally requires a substrate temperature of at least 600° C. to achieve epitaxial deposition of a silicon nitride layer. PECVD improves on basic CVD by creating a glow discharge or plasma in the reaction chamber which enables a silicon nitride layer to be formed at a much lower temperature on the order of 350° C. to 400° C.

32. Conventional silicon nitride etch stop layers are typically formed at about 350° C.-400° C., and have high concentrations of hydrogen ions which migrate to the floating gates of EEPROM cells to recombine with electrons thereon and thereby dissipate charge. This causes poor data retention as discussed above.

33. The present step of forming the silicon nitride etch stop layer 20 at a higher temperature of at least approximately 480° C. results in a substantially lower concentration of hydrogen ions in the layer 20 than in a conventional silicon nitride etch stop layer which is formed at low temperature, thereby providing substantially improved data retention.

34. A PECVD reaction chamber 22 for forming the silicon nitride layer 20 is illustrated in FIG. 11, and includes a container 24. An electrically grounded susceptor 26 is suspended in the container 24. A silicon wafer 30 including one or more dies on which semiconductor structures such as the memories 10 as illustrated in FIG. 2 are formed is supported on the susceptor 26. Lift pins 28 are provided for placing the wafer 30 on the susceptor 26. The wafer 30 is heated to a temperature of approximately 470° C. to 550° C., preferably 500° C., by a heater 32.

35. A gas discharge nozzle which is known in the art as a shower head 34 is mounted in the container 24 above the wafer 30. A gas mixture 36 which is used to form the silicon nitride layer 20 is fed into the shower head 34 through an inlet conduit 38 and discharged downwardly toward the wafer 30 through orifices 34 a. The gas 36 preferably includes NH₃, SiH₄, and N₂.

36. Radio Frequency (RF) power is applied to the shower head 34 through a power lead 40. A blocker plate 34 b is provided at the upper end of the shower head 34 to prevent gas from escaping upwardly.

37. The RF power applied to the shower head 34 creates an alternating electrical field between the shower head 34 and the grounded susceptor 26 which forms a glow or plasma discharge in the gas 36 therebetween. The plasma discharge enables the silicon nitride layer 20 to be formed at the temperature specified above.

38. In addition to a deposition temperature of at least approximately 480° C., other process conditions enhance the formation of a silicon nitride layer 20 with low hydrogen concentration. The present inventors have discovered that the qualities of the silicon nitride layer 20 are improved if the layer has relatively high density and is formed at a relatively low deposition rate.

39. This is achieved by performing deposition with low flow rates of NH₃ and SiH₄ in the gas 36, and low RF power. Preferred values for these conditions are an SiH₄ flow rate of approximately 55± 5 sccm, an NH₃ flow rate of approximately 12± 2 sccm, and an RF power of approximately 375± 10 watts.

40. The preferred conditions also include an N₂ flow rate of approximately 4,000 sccm, a pressure of 3.5± 0.2 torr, and a spacing S of approximately 375 mils (9.5 millimeters) between the shower head 34 and the surface of the wafer 30.

41. Referring now to FIG. 4, the next step of the process is to form an insulator layer 42′, preferably of tetraethylorthosilicate (TEOS) glass, over the silicon nitride etch stop layer 20. The TEOS layer 42′ is planarized as illustrated in FIG. 5 using, preferably, chemical-mechanical polishing, and redesignated as 42.

42. The remaining steps result in the formation of a tungsten damascene local interconnect structure for the memory 10. In FIG. 6, a layer of photoresist 44 is formed on the TEOS layer 42, and patterned using photolithography such that holes 44 a, 44 b and 44 c are formed above the silicide interconnect areas 18 a, 18 b and 18 c respectively.

43. In FIGS. 7 and 8, holes are etched through the TEOS layer 42 and silicon nitride layer 20 down to the interconnect areas 18 a, 18 b and 18 c, preferably using a two stage Reactive Ion Etching (RIE) process.

44. In FIG. 7, an RIE etch is performed using octafluorobutene (C₄F₈) which has a selectively high etch rate for TEOS and a low etch rate for silicon nitride. This results in the formation of vertical holes 46 a, 46 b and 46 c which extend downwardly from the holes 44 a, 44 b and 44 c of the photoresist layer 44 through the TEOS layer 42 and stop on the silicon nitride etch stop layer 20 in alignment with the interconnect areas 18 a, 18 b and 18 c respectively.

45. In FIG. 8, the photoresist layer 44 is stripped away, and a second RIE etch is performed using fluoromethane (CH₃F), which has a selectively high etch rate for silicon nitride and a low etch rate for TEOS. This results in the formation of holes 48 a, 48 b and 48 c through the silicon nitride layer 20. The holes 48 a, 48 b and 48 c are extensions of the holes 46 a, 46 b and 46 c through the TEOS layer 42, and terminate at the interconnect areas 18 a, 18 b and 18 c respectively.

46. In FIG. 9, tungsten 50 is deposited over the structure of FIG. 8. The tungsten fills the holes through the TEOS layer 42 and the silicon nitride layer 20 as indicated at 50 a′, 50 b′ and 50 c′, and ohmically contacts the interconnect areas 18 a, 18 b and 18 c respectively. The tungsten further forms on the top of the TEOS layer 42 as indicated at 50 d.

47. In FIG. 10, the top of the structure is planarized, preferably using chemical-mechanical polishing, to remove the tungsten 50 d from the TEOS layer 42. The result is independent local interconnects 50 a, 50 b and 50 c which are formed of tungsten inlaid in the TEOS layer 42 and the silicon nitride layer 20. The local interconnects 50 a, 50 b and 50 c enable the sources 14 a, drains 14 b, and control gates 14 e respectively of the transistors 14 to be electrically accessed from the upper surface of the structure.

48. In summary, the present invention overcomes the drawbacks of the prior art and provides a semiconductor structure including a silicon nitride layer etch stop layer with substantially improved data retention characteristics.

49. Various modifications will become possible for those skilled in the art after receiving the teachings of the present disclosure without departing from the scope thereof. 

We claim:
 1. A process for fabricating a semiconductor structure having a local interconnect, comprising the steps of: (a) providing a semiconductor substrate; (b) forming a semiconductor device having an interconnect area on a surface of the substrate; (c) forming a silicon nitride etch stop layer over the surface of the substrate and the device at a temperature of at least approximately 480° C.; (d) forming an insulator layer over the etch stop layer; (e) etching a first hole through the insulator layer to the etch stop layer in alignment with the interconnect area; (f) etching a second hole through the etch stop layer to the interconnect area; and (g) filling the first and second holes with an electrically conductive material which ohmically contacts the interconnect area to form the local interconnect.
 2. A process as in claim 1 , in which step (e) comprises etching the first hole using Reactive Ion Etching (RIE) with octafluorobutene.
 3. A process as in claim 1 , in which step (f) comprises etching the second hole using Reactive Ion Etching (RIE) with fluoromethane.
 4. A process as in claim 1 , in which: step (a) comprises providing the substrate of silicon; and step (b) comprises the substeps of: (b1) forming a layer of a refractory metal silicide material over the interconnect area; and (b2) reacting the silicide material with underlying silicon to form the interconnect area as a silicide.
 5. A process as in claim 1 , in which step (g) comprises filling the first and second holes with tungsten to form the local interconnect as a tungsten damascene.
 6. A process as in claim 1 , in further comprising the step, performed between steps (d) and (e), of: (h) planarizing the insulator layer using chemical mechanical polishing.
 7. A process as in claim 1 , in which step (d) comprises forming the insulator layer of tetraethylorthosilicate (TEOS) glass.
 8. A process as in claim 1 , in which step (c) comprises forming the etch stop layer at a temperature of approximately 500° C.
 9. A process as in claim 1 , in which step (c) comprises forming the etch stop layer at a temperature in the range of approximately 470° C. to 550° C.
 10. A process as in claim 9 , in which: step (c) comprises forming the etch stop layer using Plasma Enhanced Chemical Vapor Deposition (PECVD) with: an SiH₄ flow rate of approximately 55± 5 sccm; an NH₃ flow rate of approximately 12± 2 sccm; and an RF power of approximately 375± 10 watts.
 11. A process as in claim 10 , in which step (c) further comprises forming the etch stop layer with an N₂ flow rate of approximately 4,000 sccm.
 12. A process as in claim 10 , in which step (c) further comprises forming the etch stop layer at a pressure of approximately 3.5± 0.2 torr.
 13. A process as in claim 10 , in which step (c) further comprises forming the etch stop layer with a spacing between a PECVD shower head and the surface of the substrate of approximately 9.5 millimeters.
 14. A process as in claim 1 , in which step (c) comprises forming the etch stop layer to a thickness of approximately 800±50 Å.
 15. A process for fabricating a semiconductor structure, comprising the steps of: (a) providing a semiconductor substrate; (b) forming a semiconductor device on a surface of the substrate; and (c) forming a silicon nitride layer over the surface of the substrate and the device at a temperature of at least approximately 480° C.
 16. A process as in claim 15 , in which step (c) comprises forming the silicon nitride layer at a temperature of approximately 500° C.
 17. A process as in claim 15 , in which step (c) comprises forming the silicon nitride layer at a temperature in the range of approximately 470° C. to 550° C.
 18. A process as in claim 17 , in which: step (c) comprises forming the silicon nitride layer using Plasma Enhanced Chemical Vapor Deposition (PECVD) with: an SiH₄ flow rate of approximately 55± 5 sccm; an NH₃ flow rate of approximately 12± 2 sccm; and an RF power of approximately 375± 10 watts.
 19. A process as in claim 18 , in which step (c) further comprises forming the silicon nitride layer with an N₂ flow rate of approximately 4,000 sccm.
 20. A process as in claim 18 , in which step (c) further comprises forming the silicon nitride layer at a pressure of approximately 3.5± 0.2 torr.
 21. A process as in claim 18 , in which step (c) further comprises forming the silicon nitride layer with a spacing between a PECVD shower head and the surface of the substrate of approximately 9.5 millimeters.
 22. A process as in claim 15 , in which step (c) comprises forming the silicon nitride layer to a thickness of approximately 800± 50 Å.
 23. A semiconductor structure, comprising: a semiconductor substrate; a semiconductor device formed on a surface of the substrate; and a silicon nitride layer formed over the surface of the substrate and the device at a temperature of at least approximately 480° C.
 24. A structure as in claim 23 , in which: the device comprises an interconnect area; the silicon nitride layer is an etch stop layer; and the structure further comprises: an insulating layer formed over the etch stop layer; a first hole formed through the insulator layer to the etch stop layer in alignment with the interconnect area; a second hole formed through the etch stop layer to the interconnect area; and an electrically conductive material which fills the first and second holes and ohmically contacts the interconnect area to form a local interconnect.
 25. A structure as in claim 23 , in which the device comprises a memory cell having a floating element.
 26. A structure as in claim 25 , in which: the memory cell comprises a Metal-Oxide-Semiconductor (MOS) transistor; and the floating element comprises a floating gate.
 27. A structure as in claim 23 , in which the silicon nitride layer is formed at a temperature of approximately 500° C.
 28. A structure as in claim 23 , in which the silicon nitride layer is formed at a temperature in the range of approximately 470° C. to 550° C.
 29. A structure as in claim 28 , in which the silicon nitride layer is formed using Plasma Enhanced Chemical Vapor Deposition (PECVD), with: an SiH₄ flow rate of approximately 55± 5 sccm; an NH₃ flow rate of approximately 12± 2 sccm; and an RF power of approximately 375±10 watts.
 30. A structure as in claim 29 , in which the silicon nitride layer is formed with an N₂ flow rate of approximately 4,000 sccm.
 31. A structure as in claim 29 , in which the silicon nitride layer is formed at a pressure of approximately 3.5± 0.2 torr.
 32. A structure as in claim 29 , in which the silicon nitride layer is formed with a spacing between a PECVD shower head and the surface of the substrate of approximately 9.5 millimeters.
 33. A structure as in claim 23 , in which the silicon nitride layer has a thickness of approximately 800±50 Å.
 34. A structure as in claim 23 , in which: the structure is a flash Electrically Erasable Programmable Read-Only Memory (flash EEPROM); and the device comprises an erasable memory cell having a floating element.
 35. A structure as in claim 34 , in which: the memory cell comprises a Metal-Oxide-Semiconductor (MOS) transistor; and the floating element comprises a floating gate. 